The use of fluids with low boiling temperatures, or cryogens, is becoming increasingly explored and employed in the medical and surgical field. Of particular interest is the use of catheter based devices employing the flow of cryogenic fluids therein to selectively freeze or otherwise thermally affect targeted tissues within the body. Catheter based devices are desirable for various medical and surgical applications in that they are relatively non-invasive and allow for precise treatment of localized discrete tissues that are otherwise inaccessible. Catheters may be easily inserted and navigated through the blood vessels and arteries, allowing non-invasive access to areas of the body with relatively little trauma.
Catheter-based ablation systems are well known in the art. A cryogenic device uses the energy transfer derived from thermodynamic changes occurring in the flow of a cryogen therethrough to create a net transfer of heat flow from the target tissue to the device, typically achieved by cooling a portion of the device to very low temperature through conductive and convective heat transfer between the cryogen and target tissue. The quality and magnitude of heat transfer is regulated by the device configuration and control of the cryogen flow regime within the device.
A cryogenic device uses the energy transfer derived from thermodynamic changes occurring in the flow of a refrigerant through the device. This energy transfer is then utilized to create a net transfer of heat flow from the target tissue to the device, typically achieved by cooling a portion of the device to very low temperature through conductive and convective heat transfer between the refrigerant and target tissue. Structurally, cooling can be achieved through injection of high pressure refrigerant through an orifice. Upon injection from the orifice, the refrigerant undergoes two primary thermodynamic changes: (i) a depressurization (adiabatic) and temperature drop through positive Joule-Thomson throttling, and (ii) undergoing a phase change from liquid to vapor, as the fluid absorbs heat. The resultant flow of low temperature refrigerant through the device acts to absorb heat from the target tissue and thereby cool the tissue to the desired temperature.
Presently available cooling systems typically operate based upon a set coolant flow rate or coolant pressure in the catheter or medical device that is used to reach a desired temperature for treatment. A measurement of the flow rate or pressure may be used in a feedback loop to control a pump or other component controlling the actual coolant flow. However, depending on the thermal load experienced by a particular device as well as the particular target temperature trying to be reached, there can be significant thermal variations for a predetermined flow or pressure value at different portions of a medical device having fluid flow therethrough. For example, for a fixed flow rate and a low thermal load on a device, a flow rate may become excessive, resulting in a portion of the circulating fluid failing to change phase from a liquid/solid to a gas, and thereby reducing the overall thermal efficiency and affect on the surrounding tissue. Moreover, at a high heat load, a set flow rate may not sufficiently provide a treatment area on the device having the desired temperature, i.e., the temperature may vary drastically from one location to the next despite the proximity of the two locations because of the temperatures trying to be achieved and the thermal energy/load surrounding a particular device. As a result, the actual tip or device temperature may be different than a target temperature correlating to a set flow or pressure due to thermal variations at the treatment site.
Accordingly, it would be desirable to provide an improved apparatus and method of monitoring and controlling the circulation of a coolant through a medical device such as an intravascular catheter.